Several methods are known for regulating the output power of a power supply. In switched mode power supplies, regulation by means of pulse width modulation is the most common method used. Another method of regulating output power is the use of a tuned circuit in series with an AC voltage source. The tuned circuit includes an inductance and a capacitor and has a resonance frequency associated with it. FIG. 1 shows the attenuation curve of a series connection of an inductance L and a capacitance C acting as an LC filter as a function of frequency. The resonance frequency Fres of the inductance L and the capacitance C is given by Equation 1 for the simplest circuit that can be used for power regulation to a resistance load employing a resonant circuit (an LC filter): EQU Fres=1/(2.pi.LC) (Eq. 1)
By definition, the attenuation at the frequency Fres is 1, meaning that all energy passes through the circuit and there is maximum power. In the regions adjacent the resonance frequency Fres, only a portion of the energy passes through.
Power output to a load resistance from an LC filter may be regulated by adjusting the correspondence between the frequency of an AC source and the frequency of the resonant circuit formed of the LC filter with inductance L and capacitance C. The correspondence between the frequency of an AC source and the frequency of a resonant circuit may be adjusted by either changing the frequency of the AC source closer to or farther away from the resonance frequency of the resonant circuit, or, in the case of a fixed frequency AC source, by changing the resonance frequency of the resonant circuit closer to or away from the frequency of the AC source. FIG. 2 illustrates attenuation curves for power regulation accomplished by changing the resonance frequency of a resonant circuit closer to or farther away from the AC source. More particularly, in FIG. 2, Fac is the frequency of the AC source signal. Fres.1 is the attenuation curve of an LC filter, as in FIG. 1. The point A where the frequency of the AC source Fac crosses the attenuation curve for the LC filter having a resonance frequency Fres.1, illustrates the amount of attenuation of the frequency of the AC source Fac that is achieved by the LC filter. In the situation illustrated, the amplitude of the signal after the LC filter is reduced by the LC filter to about 15% of the amplitude of the frequency of the AC source Fac as shown by point A.
However, if the curve of the resonance frequency is moved in the direction of the frequency of the AC source Fac, the attenuation of the frequency of the AC source Fac by the LC filter changes, and the voltage output amplitude rises, increasing the power output. The curve of the resonance frequency may be moved in the direction of the frequency of the AC source Fac by changing one or both of the values of the inductance L and the capacitance C in accordance with Equation 1 above. In FIG. 2, this shift is illustrated by moving the curve of the resonance frequency Fres.1 in the direction of the horizontal arrow to a new resonance frequency Fres.2. The point B where the frequency of the AC source Fac crosses the new attenuation curve shows that the signal after the LC filter is increased to about 90% of the amplitude of the frequency of the AC source signal Fac as shown by the vertical arrow. Thus, the output voltage and hence the output power are increased. By changing the resonance frequency of the inductance L and the capacitance C of the resonant circuit, the output power delivered by the power supply to a load resistance can be regulated.
A second method of power regulation is achieved by changing the frequency of an AC source closer to or farther away from the resonance frequence of a resonant circuit. FIG. 3 illustrates this method graphically where the resonance frequency Fres is kept constant, while the graph of frequency of the AC source is varied from Fac1 to Fac2 in the direction of the horizontal arrow. For the first frequency of the AC source Fac1, the point of crossing of the attenuation curve and Fac1 results in an amplitude after the LC filter of about 15% of the amplitude of the frequency of the AC source Fac. When the frequency of the AC source is decreased as indicated by the horizontal arrow to the value Fac2, the attenuation of the frequency of the AC source by the LC filter changes and the output amplitude rises. At the new frequency of the AC source Fac2, the point B of crossing of the attenuation curve increases to about 90% of the amplitude of the frequency of the AC source as shown by the vertical arrow. Accordingly, the output voltage and the output power to a load resistance is increased. By changing the frequency of the AC source from Fac1 to Fac2, power can be regulated.
FIG. 4 shows a prior art power regulator for supplying power to a resistance load which performs power regulation by a resonant circuit which is more complicated than just the LC filter discussed above. More particularly, the power regulator 10 illustrated in FIG. 4 has an AC source 11, a resonant circuit 12, an isolation transformer 13, rectifying means 14, and an output across terminals 18 and 19 for connecting to the load resistor R.sub.load 20. The resonant circuit 12 includes an inductance L and a capacitance C. The rectifier means 14 is a rectifier and includes a diode circuit made up of diodes 15 and 16, and a smoothing capacitor 17. A circuit in accordance with prior art FIG. 4 is disclosed in U.S. Pat. No. 4,930,063, issued to Henze et al. on May 29, 1990, and includes a variable inductor for the inductance L of the resonant circuit. Accordingly, in the Henze et al. regulator for a power supply, the resonance frequency is varied by varying the inductance of the resonant circuit in order to regulate power. An attenuation curve similar to that of FIG. 1 may be obtained for the prior art circuit in the Henze et al. Patent, and power regulation may be obtained by changing the resonance frequency of the resonant circuit in a manner similar to that graphically illustrated in FIG. 2.
Several disadvantages are associated with the prior art circuits for power regulation employing resonant circuits. As illustrated in the attenuation curves of FIGS. 1-3, the attenuation curve for each circuit approaches but never equals zero amplitude of the frequency of the AC source. The output power can never equal zero. This is because the attenuation of the LC filters of the prior art circuits would have to be substantially infinitely high in order to regulate to a voltage lower than the AC source voltage when the current at the output of a circuit is equal to essentially zero and the resistance of the load connected to the circuit is substantially infinite. A minimum power output can only occur at substantially an infinitely high source frequency or at substantially an infinitely large inductance L. Further, the diodes in the rectifier result in significant recovery losses due to significant instantaneous reverse voltages immediately after recovery. In prior art circuits, the voltages across the diodes of the rectifier are subject to sharp transitions. Further, the prior art circuits are subject to parasitic effects from the leakage inductance of the isolation transformer, the winding capacitances of the isolation transformer, and capacitances associated with the diodes of the rectifier. Further, isolation transformers are associated with power dissipation due to the resistance of the copper windings referred to as copper losses of the windings.
There is therefore a need for a power regulator that regulates power by adjusting the correspondence between the frequency of an AC source and the resonance frequency of a resonant circuit which permits power regulation between a maximum value and a minimum value of zero. Further, there is a need for a power regulator employing a resonant circuit that reduces the recovery loss in diodes employed in the rectifier. Additionally, there is a need for a power regulator that regulates power by employing a resonant circuit that reduces or eliminates the parasitic effects due to transformer leakage, copper losses of transformer windings, unwanted winding capacitances associated with a transformer, and unwanted capacitances associated with diodes in the rectifier means.